It is possible to get SystemError on import (see attached archive).

$ ./python -c 'import package.module1'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/serhiy/py/cpython/package/module1.py", line 3, in <module>
    from . import module2
SystemError: Parent module 'package' not loaded, cannot perform relative import

SystemError means a programming error in interpreter core or extension. It is comparable to an assert in C code, but without immediate crashing. Since this situation can be provoked by user code, it should be replaced with other exception (KeyError, RuntimeError, ImportError, etc).